True about cornea –a) Power is 43 Db) Majority of refraction occur at air – tear interfacec) With the rule astigmatism is present because vertical meridian more steep than horizontal meridiand) Spherical structuree) Refractive index 1.334
**Core Concept**
The cornea, a transparent layer at the front of the eye, plays a crucial role in refracting light and contributing to the eye's overall refractive power. Its unique structure and properties allow it to facilitate this process.
**Why the Correct Answer is Right**
The majority of the eye's total refractive power is attributed to the cornea, with an average power of approximately 43 diopters (D). This is due to its steep curvature and the significant difference in refractive indices between air (1.00) and the cornea (1.376). As a result, most of the refraction occurs at the air-tear interface, which is the boundary between the cornea and the air. The cornea's structure is not spherical but rather a prolate spherocylindrical shape, which contributes to its refractive properties.
**Why Each Wrong Option is Incorrect**
* **Option A:** The correct power of the cornea is approximately 43 D, not 43 D, making this option incorrect.
* **Option C:** In with-the-rule astigmatism, the horizontal meridian is more steep than the vertical meridian, not the other way around. This results in blurred vision at near and far distances.
* **Option D:** The cornea is not a spherical structure but rather a prolate spherocylindrical shape, which is essential for its refractive properties.
* **Option E:** The refractive index of the cornea is approximately 1.376, not 1.334.
**Clinical Pearl / High-Yield Fact**
The cornea's unique refractive properties are essential for clear vision. Understanding its structure and function can help diagnose and treat various eye disorders, such as astigmatism and keratoconus.
**Correct Answer:** B. Majority of refraction occur at air – tear interface